Non-melt-processable tetrafluoroethylene [TFE] polymers have so far been used in various fields of application. For example, porous bodies obtained by extrusion molding of such a TFE polymer and the subsequent stretching are excellent in air permeability and are used as clothes, tents and industrial precision filters, among others.
In cases where a TFE polymer is used in the form of porous bodies, in particular industrial filters, a high level of membrane homogeneity is required, and the demand not only for high-level membrane homogeneity but also for lower pressure losses is growing. Therefore, works are under way to develop a TFE polymer capable of giving a high level of membrane homogeneity.
For the purpose of providing polytetrafluoroethylene [PTFE] species excellent in strength and capable of being suitably subjected to a stretching procedure following paste extrusion molding, PTFE species having a standard specific gravity of not higher than 2.160 and a tensile strength at break of 32.0 to 49.0 N, for instance, have been proposed (cf. e.g. Patent Document 1). However, there is no description about membrane homogeneity in biaxial stretching in which membrane homogeneity can hardly be attained.
Patent Document 1: Japanese Kokai Publication 2002-201217